1. Field of the Invention
The invention relates to a measurement device for detecting and gripping sheets in a region of the front lays of a paper sheet processing machine with means for accelerating the sheets aligned with their leading edges at the front lays to a higher velocity sufficient for transfer to a paper sheet processing unit. The invention further relates to a method for scanning for the presence of sheets in the region of the front lays of a paper sheet processing machine with means for accelerating the sheets aligned along their leading edges at the front lays to a higher velocity necessary for transfer to a paper sheet processing unit. Finally, the invention relates to a gripper device in paper sheet processing machines, in which paper sheets that are aligned at the front lays along their leading edges are received by grippers and are accelerated by the grippers for transfer to a paper processing unit.
2. Description of the Related Art
It has been known in the art of paper sheet processing machines to entrain paper sheets with pre-grippers after they are aligned along their leading edges at the front lays. The gripper transfers the sheets to receiving devices of a paper processing unit disposed downstream as seen in the transport direction, for instance the clamping bar of an impression cylinder of a sheet-fed rotary offset printing machine. The purpose of the pre-grippers is to accelerate the sheets from an alignment position at the front lays, to a velocity which is adapted to the processing speed of the paper processing unit. In the case of high accuracy requirements in the paper processing unit, it is desirable to keep the acceleration of the sheets as low as possible. On the other hand, the acceleration to very high speeds, for instance in fast sheet-fed rotary offset print, is very much limited due to the spatially limited acceleration path.
The time available for acceleration is further substantially reduced by a reaction time of the system and by a sheet alignment safety time period. The sheet alignment safety time period is the time span which elapses before it is ascertained that a paper sheet is present at the front lays for transfer by the pre-gripper. Such safe alignment has been heretofore determined by means of photo-electric sensor devices disposed upstream and downstream, respectively, of the front lays. The sensor devices are disposed below the transport plane and they scan a large region above the sensor location. The sensors by themselves can thereby only ascertain that some paper sheet is present somewhere in a relatively large area above the sensor. The sensor which is disposed upstream of the front lays can merely determine that a part of a paper sheet is located in the region of the front lay. The upstream sensor cannot detect whether or not it is a paper sheet aligned at the front lays or if it is a paper sheet which has shot over the front lay. Also, in the case of long paper sheets, it may even be a trailing edge of a preceding sheet. Only the second, downstream sensor can deliver necessary information when, after a predetermined time window, it does not register a sheet. The information is whether or not the sheet registered by the first sensor is present in an aligned position and ready for transfer by the pre-grippers. The predetermined time window allocated to the second sensor undesirably limits the sheet length to be processed. Only such a length of paper sheets can be processed which, after the trailing edge of the previous sheet has left the second sensor, allow the second sensor, through the predetermined time window and the time span until the sensor is again covered up by the leading edge of the next sheet delivered by the pre-grippers, to register a non-cover.
Furthermore, the interpretation of the signals of two sensors establishes a delay which is to be subtracted from the time available for the acceleration. The acceleration of the sheets, if made too great, causes detrimental alignment shifts in the paper sheets as well as vibrations and feedback with other machine parts.